Monitoring of flames in industrial or utility power plants to assure satisfactory and efficient operation and obtain an indication or control signal for shut-down in the event of malfunction is standard practice. Various forms of monitoring systems are known including various forms of photocell sensor systems which attempt to detect the presence of the flame and produce an indication or control signal when the flame goes out or becomes unsatisfactory for any reason.
In the past, flame scanners or monitors have relied upon flicker characteristics of the flame to produce an alternating signal component superimposed on a background signal level representing average brightness. An example of circuits of this type is found in the U.S. Patent to Cade U.S. Pat. No. 2,811,711. A later form of flame monitor which uses different frequency characteristics of the flame to develop control signals was the patent to MacDonald U.S. Pat. No. 4,039,844.
These and other prior art flame monitor circuits which relied upon the frequency characteristic of the flame have provided satisfactory operation, particularly with reference to flames from burning oil or gas as fuel where the flame is relatively steady and the radiation therefrom can be directly observed. Some flames from these fuels and the flames which are produced in large power plant installations by burning pulverized coal have different characteristics and in certain respects present problems not found in monitoring well behaved oil and gas flames.
One of the problems which is present in large industrial or utility power plants is the presence of multiple burners which feed the same fire box with the internal volume occupied by a large fire ball and the walls of the firebox becoming so hot that the walls themselves provide radiant energy which will energize a photodetector. In such systems the ability to detect when an individual burner flame has gone out is complicated by the fact that the photoresponsive sensor in the absence of flame in the burner which it is monitoring will see the fire ball or the radiation from the fire box wall and thus its response will not necessarily fall to zero merely because the flame it is monitoring has gone out. It is for this reason that the various flicker frequency characteristics of the flame and other spectral bases for discriminating an actual burner flame from other radiation present in the environment have been researched and employed for many years.
As noted, there are certain flames which are more difficult to monitor than others. One such type of flame is the result of burning pulverized coal of which there are many types. For example, stone coal, brown coal and lignite all have different firing characteristics and produce characteristic flames when supplied to a burner in pulverized form to produce the flame. Such fuels are generally opaque and do not instantly ignite as the nozzle discharges them into the fire box at the burner nozzle. When the pulverized coal does ignite a large increment of luminousity is generated at an area which just previously was occupied by opaque coal dust. Thus to a photodetector which is observing such an area there is a large fluctuating component in the brightness of the area and if the photo detector responds at high speed the signal that is obtained will be a rapid fluctuation relative to an average level representing the average brightness of the observed flame. Extensive work has been done in an attempt to utilize frequency discrimination to monitor flames having these high level fluctuations in order to find a characteristic which can be relied upon to indicate the presence of a proper and well behaved flame and the immediate indication that the flame has disappeared or degraded to such an extent that it needs attention.
In all of the flame monitoring systems for industrial power plants the energy content of the fuel is such that an immediate and reliable indication of flame-out is imperative since the discharge of unburned fuel from a burner that has had the flame extinguished into the fire box creates a highly dangerous condition which if it remains unchecked for any length of time is likely to result in a dangerous explosion.